Papermaking is a sophisticated operation involving massive and very expensive machines. Papermaking machines are increasingly running at higher speeds, meaning that their overall efficiency must be very high, and in particular, the efficiency of their sub-components must also be very high. Papermaking requires, inter alia, the drying of a pulp fiber solution by passing the pulp over a large number of drying rolls. The operation requires that the paper web be supported on a sheet, felt, wire, fabric or other support means as the web is advanced through the papermaking machine, and water is removed therefrom. The paper web moves from the wet end, at the beginning of the machine, to the dry end, located at the end thereof, where at the finished product (i.e. the paper) is reeled.
Papermaking machines may have a number of configurations. One configuration in particular is the one where the papermaking machine comprises two rows of heated drying cylinders or rolls over which the web (i.e. the wet paper being dried) is supported and is made to be in contact with. This contact of the paper web and the heated drying rolls is primarily responsible for the drying of the web. Felt rolls are provided intermediate the two rows of drying rolls. The felt rolls support felts that are also carried through the machine. In order to dry the paper web, it is desirable that the web be in direct contact with the drying rolls for as long as possible. However, at given intervals, the paper web is unsupported as the paper web passes from one drying roll to the other. Traditional drying roll and felt roll arrangements comprise placing of the felt roll intermediate the axis of rotation of the drying rolls, thus substantially halfway between two adjacent drying rolls. Inherent in the configuration and disposition is the fact that the paper web is unsupported at given intervals such as when one of the felts separates from the paper web to pass over a felt roll.
In order to increase the speed of papermaking machines and, at the same time, reduce the length of unsupported paper web, the disposition and configuration of the felt rolls where improved by moving the axis of rotation of the felt rolls backwards, i.e. towards the wet end of the machine. The resulting effect of moving the felt rolls backwards has been the creation of what is known as offset pockets. An offset pocket may be roughly described as the enclosed space limited by the felt roll, a drying roll either above or below it, and the paper web on both sides of the felt roll. The creation of offset pockets has resulted in the felt and the paper web being in contact longer, thus minimizing the length of unsupported paper web as it passes between adjacent drying rolls.
The ventilation of offset pockets is more difficult than with traditional configurations. Since one of the fundamental aspects of papermaking is the drying of the paper web, increased difficulty in ventilating the offset pockets may therefore be counter productive and limit the speed increases of papermaking machines. The difficulty created with the presence of the offset pockets is due in part to the elimination or reduction of the draw of felt without the paper web joined thereto, upstream of the felt roll and through which heated dry air was generally introduced there through. In an offset pocket, the felt at the upstream part of the pocket from a felt roll supports the paper web. Since the paper web is substantially impermeable to air, air cannot be blown through the felt and then into the pocket as it used to be. As a result, this decreases the air flow into the pocket, with a resultant decrease in the efficiency of the ventilation and drying of the paper web.
Now, in accordance with the present invention, it has been found that there is one area through which air may be introduced is on the dry end side of the pocket, between the point where the felt leaves the felt roll and the point where the felt comes into contact with the next drying roll. This allows to provide a ventilator and corresponding method which, with properly disposed orifices, can very suitably ventilate offset pockets.